1. Field of Invention
The present invention relates to alloys of beryllium and magnesium. More particularly, the invention is a method of making alloys of magnesium containing beryllium and forming them into useful structural products.
2. Brief Description of the Prior Art
Currently, there are no known practical or useful structural alloys of beryllium and magnesium. Available information in the art reports the production of MgBe.sub.13, a brittle intermetallic compound which cannot be used in any known practical manner (Stonehouse, Distribution of Impurity Phases, Beryllium Science & Tech., 1979, Vol. 1, pages 182-185). Commercially available beryllium ordinarily contains under 1000 ppm by weight magnesium as a residual component used in reducing BeF.sub.2 in the normal refining process, and even this trace amount of magnesium is present as the intermetallic compound, MgBe.sub.13 (Walsh, Production of Metallic Beryllium, Beryllium Science & Tech., 1979, Vol. 2, page 8).
Early research conducted at the Los Alamos Scientific Laboratory by F. H. Ellinger's group showed that reduction of BeF.sub.2 with molten magnesium produced the intermetallic compound MgBe.sub.13, and dilution of a pre-alloy of aluminum-beryllium with magnesium resulted in an overall mass largely in the form of MgBe.sub.13 dendrites which was 34.4% beryllium (Elliott, Preparation and Identification of MgBe.sub.3, Metallurgy and Ceramics, 13th Ed., 1958, pages 1-10). The British confirmed the shortcomings of intermetallic MgBe.sub.13, made with porous beryllium powder infiltrated with molten magnesium, for their brittleness (Jones, Preparation of Beryllium-Magnesium Alloys by Powder Metallurgical Methods, United Kingdom Atomic Energy Authority Memorandum, 1961, AERE M 828). Jones observed that such alloys had structure consisting of a network of MgBe.sub.13 surrounding grains of beryllium which contributed to the brittleness and high hardness.
The use of beryllium as a protective oxide during the processing of magnesium-rich master alloys is known. Such beryllium is used to prevent oxidation of the magnesium during transit and distribution to downstream processors. For instance, Brush Wellman Inc. of Elmore, Ohio, produces and distributes magnesium-rich pellets using 5% or less beryllium. Such pellets are made by hot-pressing powdered magnesium alloys together with powdered beryllium. The residual beryllium level in the downstream processors' final magnesium product is less than 0.01%.
Conventional semi-solid processing or thixo-forming of metals is a manufacturing method which takes advantage of low apparent viscosities obtained through continuous and vigorous stirring of heat-liquified metals during cooling (Brown, Net-Shape Forming Via Semi-Solid Processing, Advanced Materials & Processes, Jan. 1993, pages 327-338). Various terminology is presently used to describe semi-solid processing of metals to form useful articles of manufacture, including such terms as rheo-casting, slurry-casting, thixo-forging and semi-solid forging. Each of these terms is associated with variations in the steps during semi-solid processing or in the types of equipment used.
Generally, semi-solid processing is initiated by first heating a metal or metals above their liquidus temperatures to form molten metal or alloy. Various methods known in the art are used to introduce shear forces into the liquified metals during slow cooling to form in situ, equiaxed particles dispersed within the melt. Under these conditions, the metals are said to be in a "thixotropic" or semi-solid slurry state. Thixotropic slurries are characterized by non-dendritic microstructure and can be handled with relative ease in mass production equipment allowing process automation and precision controls while increasing productivity of cast materials (Kenney, Semisolid Metal Casting and Forging, Metals Handbook, 9th Ed., 1988, Vol. 15, pages 327-338).
Non-dendritic microstructure of semi-solid metal slurries is described in Flemings U.S. Pat. No. 3,902,544. The method disclosed in this patent is representative of the state of the art which concentrates on vigorous convection during slow cooling to achieve the equiaxed particle dispersion leading to non-dendritic microstructure (Flemings, Behavior of Metal Alloys in the Semisolid State, Metallurgical Transactions, 1991, Vol. 22A, pages 957-981).
Published research prior to the present disclosure has focused on seeking an understanding of the magnitude of forces involved in deforming and fragmenting dendritic growth structures using high temperature shearing. It was discovered that semi-solid alloys displayed viscosities that rose to several hundreds, even thousands of poise depending on shear rates (Kenney, Semisolid Metal Casting and Forging, Metals Handbook, 9th Ed., 1988, Vol. 15, page 327), and that the viscosity of a semi-solid slurry, measured during continuous cooling, was a strong function of applied shear forces, such measured viscosities decreasing with increasing shear rate (Flemings, Behavior of Metal Alloys in the Semi-Solid State, ASM News, Sept. 1991, pages 4-5).
Thus, subsequent commercial exploitation focused on developing different ways to agitate liquified metals, before or substantially contemporaneous to forming in a die, to achieve the roughly spherical or fine-grained microstructure in semi-solid slurry. Two general approaches to the forming process developed--(1) rheo-casting, in which slurry is produced in a separate mixer and delivered to a mold; and (2) semi-solid forging, in which a billet is cast in a mold equipped with a mixer which creates the spherical microstructure directly within the mold.
For example, Winter U.S. Pat. No. 4,229,210 discloses a method of inducing turbulent motion in cooling metals with electro-dynamic forces using a separate mixer, while Winter U.S. Pat. Nos. 4,434,837 and 4,457,355 disclose a mold equipped with a magneto-hydro-dynamic stirrer.
Various methods for agitating or stirring have been developed to introduce shear forces in the cooling metals to form semi-solid slurry. For example, Young U.S. Pat. No. 4,482,012, Dantzig U.S. Pat. No. 4,607,682 and Ashok U.S. Pat. No. 4,642,146 all describe means for electromagnetic agitation to produce the necessary shear forces within liquified metals. Mechanical stirring to produce the desired shear rates are described in Kenney U.S. Pat. No. 4,771,818, Gabathuler U.S. Pat. No. 5,186,236 and Collot U.S. Pat. No. 4,510,987.
Application of currently known semi-solid processing technology to alloys of magnesium containing beryllium is impractical because the melting point of beryllium is in excess of 1280.degree. C. At such temperatures and under standard atmospheric conditions, magnesium vaporizes at a boiling point of 1100.degree. C. (Elliott, Preparation and Identification of MgBe.sub.13, Metallurgy and Ceramics, 13th Ed., 1958, pages 1-10). Currently known thixo-forming processes would require an initial high temperature liquidization of beryllium at above 1200.degree. C. which would cause magnesium to boil away. This, in fact, is the commercially available process now used to remove magnesium impurities from beryllium during refining (Stonehouse, Distribution of Impurity Phases, Beryllium Science & Tech., 1979, Vol. 1, page 184).
The present disclosure describes solutions to the problems described above for making alloys of magnesium containing beryllium and further introduces a novel improvement in semi-solid processing for metal alloys.